Hmmm, there is something in my head, that couldn't be resolved. So someone help me!
Thinking about colecting the light from faraway stars, I came to think about the best lens to collect those valuable few photons, that reach the earth. My conclusion: take the lens with the absolute largest aperture as opposed to the relative aperture that is designated on the lens.
What do I mean by "absolute aperture"? Ok quite simply, the diameter of the front-lens. Example: A 400mm/f8 has an "entry pupil" of 5cm, a 50mm/f2 an entry pupil of 2.5cm.

How did I come to this conclusion? Well, it's all about collecting photons, isn't it? And assuming a star is so far away, that with optimal focus it just "fills" 1 photocell whether with a 50mm or a 400mm than the larger entry pupil of the 400/f8 should collect 4 times the photons than the 50mm/f2.

You are right. For stars, providing lens quality and diffraction aren't limiting, aperture is the most important factor as it determines light gathering power. As usual, even with a perfect lens if you operate at too high an f-number diffraction will become an issue.

For extended objects like nebulae (or stars so close together they can't be individually resolved) f-number is important. For a given aperture the smaller the f-number the brighter the nebula is on the sensor. If sensors counted every photon and had no noise that wouldn't matter but in practice a small f-number is needed to get the signal from faint nebulae above the sensor noise.

Obviously there are trade-off's involved. There is no point in using a hypothetical 10mm f/1.2 lens as the small aperture restricts the stellar limiting magnitude and the short focal length means that any nebulae in the field of view will be too small on the sensor to be of much use.

An optic capable of f/1.2 (the Canon EF 85mm f/1.2 for example) would be brilliant at showing nebulae and the Milky Way etc. but, quite apart from the inevitable optical aberrations from such an extreme lens when wide open, f/1.2 might sometimes be too fast as stray background light from the sky (from the Moon or as a result of many local authorities trying to light up the universe rather than the streets) may manifest itself so quickly that the stellar limiting magnitude is quite poor. At least with camera lenses it is easy to stop the lens down.

This isn't just a matter of theoretical nicety either. Take the Canon EF 85mm f/1.2 and the EF 135mm f/2 and ignore considerations like field of view. The 135mm lens has nearly the same light gathering power(10% less in fact) but at f/2 is two stops slower than the 85mm lens. However, the 85mm lens costs nearly twice as much. Assuming I had pockets deep enough to afford the 85mm I would still have to be convinced that I would be able to use it wide open for more than a few nights each year. I am still trying to work that issue out.

Hey, congrats Bob on your 500th post! You are now entitled to "Senior serious contributor of 2007" so watch out that you don't sink below your avarage of 1.40 posts per day, otherwise this title can be nullified and voided next year But you confused me by using "light gathering power" and "aperture" so I understand you would prefer the 85/1.2 (entry pupil=71mm) over the 135/2 (entry pupil=68mm). That is consistent with what I thought. But would you also prefer a 85mm/1.4 (entry-pupil=61mm) over the 135/2.0?
So what is more important: the (absolute) diameter of the "entry-pupil" or the (relative) aperture???

Sorry for the confusion: I was reverting to terms more commonly used in the telescope world. In those terms, as you inferred, aperture equates with the diameter of the objective lens or main mirror. They are just photon collectors and as such the diameter determines the light gathering power of the telescope. I should have been more careful as in the camera world aperture is, I think, more commonly equated with f-number.

Yeah, that was the reason, I used the term "(relative) aperture" which I equated to f-number, just to make sure that it is not confosed with the "(absolute) aperture" or "entry-pupil" (which in german is a perfect optical term - just don't know whether you can use/understand it in english ).
So, apart from wording issues, we are in perfect agreement!

Sorry to dredge up an old thread, but could someone look at these and tell me perhaps what I'm doing wrong?

5", f/5.6, ISO 100

30", f/5.6, ISO 400

30", f/5.6, ISO 1600

Camera is a Canon Rebel XT, 18-55mm lens. I'm wondering if perhaps I should change the metering, and if there's too much ambient light around my house for this as well? I'm also wondering if I had my focus "backwards"...too close in instead of to infinity (still getting used to my camera). Any help is much appreciated! Thanks!

Gordon, thanks! The moon shot was also corrupted, I think, by the old saw mill behind our house that has a sodium-arch lamp illuminating their yard...and my house...and my block...etc. I may have to go out of town a ways to get a good shot.

Since I'm new at this whole thing...the smaller the "f" number, the larger the opening (aperture) and the higher the ISO, the faster the "exposure" (since it's digital, that's the time sensitivity of the sensor, right)?

Gordon is probably still asleep so I'll step in, if I may. You are right on both counts regarding f-number and ISO. So for shots of the stars, especially when unguided, you need to use a high ISO number (I see you used 1600) and the smallest f-number (biggest aperture) you can. Gordon's tip about focussing is a good one but to get really crisp stars you may still need to do a little trial and error adjustments by making very small tweaks on the manual focus with subsequent trial shots. The key is to be methodical and to examine the result of each trial shot on the LCD by zooming in on a star. You will likely spend far more time doing this than you do taking the final exposure.

Sodium street lights are a right royal pain. There are filters you can buy which cut out a lot of the glare (see here) but they aren't cheap so I think your idea of finding a slightly better location is a good one. Be warned though - once you start getting a few good shots this can become an addictive pastime.

Bob, thanks for the tips! Yeah, I'm already thinking about what I'll be able to shoot tonight, what settings I'll need, etc. Doing some traveling this weekend, so hopefully I'll be able get some time to shoot then, too.

I tried astrophotography for the first time a few minutes ago and came up with really weird results. The shots looked exactly like I took them during the day albeit with a slight orange hue. I used a 7sec exposure at f/2.8, ISO 100.

The clouds ended up a slight off-white colour, I'm guessing this is because of the street lights but the weird bit was the sky. It was blue, just like during the day.

My guess is moonshine. No, not the imbibable variety. Moonlight is is, after all, just dim sunlight. I'm a little surprised that 7sec exposure at f/2.8, ISO 100 was enough to fully show this so maybe the white balance adjustment in the camera was also in play.

I remember many years ago (20+) doing a night shot with an OM2 using auto-exposure. That camera directly sensed reflected light from the film during the shot to calculate the correct exposure. Can't remember the exposure settings of course but I was similarly surprised to see a blue sky when I got the film back. It was actually quite a nice effect as it was of a church and the biggest clue that something was up was a scattering of stars in an otherwise clear blue sky.

Thanks for the reply Bob. The moon was hidden behind the clouds so I thought that it wouldn't make much of a difference but I guess it did after all. I'll pick a moonless night next time I try. I used the "direct sunlight" WB setting which I assume is the closest to the daylight setting that Gordon recommended?

It's a shame that there was so much cloud cover, if not for the clouds I might have been able to get more than 4 stars...